Speak Freely for Unix: sfecho

by John Walker

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SFECHO(1)                USER COMMANDS                  SFECHO(1)

     sfecho - Speak Freely remote echo server

       sfecho [ -du ] [ -pport ] [ -vtimeout ]
              [ -zdrop%,shuffle%,depth ]

       sfecho  is  a server program which receives sound from re­
       mote hosts in the same manner as sfspeaker, but instead of
       playing	sound  on the workstation's audio output, buffers
       it in memory for ten seconds and then  retransmits  it  to
       the  sending  host.   This allows remote users, even those
       with half-duplex audio hardware, to experiment  with  dif­
       ferent  compression  and encryption modes to optimise end-
       to-end performance without tying up a person on the  other
       end of the connection.

       sfecho  can echo sound in RTP and VAT protocols as well as
       Speak Freely's native protocol.	To permit this,	 messages
       on  both	 the  specified port and the next higher numbered
       port are monitored, queued, and retransmitted.

       An echo server can invite connections  by  publishing  its
       existence  on  one  or  more  directory	servers.  See the
       ``Look Who's Listening'' section in the	sfspeaker  manual
       page  for details on how to set the environment variables.

       -d     Enables debug output from sfecho.

       -pport Causes sfecho to listen on the specified port  num­
	      ber  instead of the default port specified by ``IN­
	      TERNET_PORT'' in the Makefile.  To comply with  the
	      RTP  standard,  the  port	 specified should have an
	      even number; the next higher numbered port is  used
	      for RTCP control messages.

       -u     Prints how-to-call information.

	      When sfecho receives a packet from a host it hasn't
	      heard from in timeout seconds, it will  attempt  to
	      find  the host name and print a message on standard
	      error noting the new connection and  what	 compres­
	      sion mode is being used.	If the host name can't be
	      found, the numeric  IP  address  is  given.   After
	      timeout  seconds	of inactivity a message is issued
	      indicating the host is  idle.   If  no  timeout  is
	      specified, 180 seconds is used.

	      sfecho  will simulate an imperfect Internet connec­
	      tion by randomly dropping (discarding)  drop%  per­
	      cent  of the packets it receives, then shuffling in
	      order shuffle% percent  of  the  remaining  packets
	      with  a  maximum reordering of depth packets.  This
	      option should never be used on  a	 production  echo
	      server;  it is intended to simulate bad connections
	      to permit testing various error-tolerant algorithms
	      to cope with them.

       If  sfecho runs out of memory to buffer sound awaiting re­
       transmission, packets are discarded without warning.

       By default, sfecho listens on Internet port  number  2074,
       the  same  as  sfspeaker, and hence cannot run at the same
       time on the same machine as sfspeaker.  You can specify	a
       different  port	for  sfecho  with the -p option, but many
       users have difficulty connecting to servers with	 nonstan­
       dard  port numbers, so this isn't a terribly good idea for
       public servers.	If your server is intended  primarily  to
       echo  messages  in  RTP	protocol, you may want to specify
       port 5004, the default  port  for  RTP  specified  in  RFC
       1890.	 Many VAT compatible applications default to port

       The implementation  of  MD5  message-digest  algorithm  is
       based on a public domain version written by Colin Plumb in
       1993.  The algorithm is due to Ron Rivest.  The	algorithm
       is described in Internet RFC 1321.


Back to Speak Freely for Unix

by John Walker
August 28, 2002